The present invention relates generally to the synthesis of carbamate and, more particularly, to the synthesis of carbamate and isocyanate through the heterogeneous oxidative carbonylation of amines using copper-based catalysts and/or Group VIII metal-based catalysts.
Isocyanates are important intermediates in the manufacture of pesticides, polyurethane foam plastics, synthetic leather, adhesives, and coatings. Currently, isocyanates are commonly produced by a phosgene-amine reaction, although considerable efforts have been devoted to developing a new phosgene-free route. An alternative, phosgene-free route is desirable because phosgene is highly toxic, making the production process inherently unsafe and significantly increasing the manufacturing cost of isocyanates.
One promising alternative approach that has been the subject of research in recent years involves the oxidative carbonylation of amines to carbamate followed by thermal decomposition of the carbamate to isocyanate. So far, Group VIII metals and/or copper-based catalysts with halide promoters have reportedly been the most active catalysts for oxidative carbonylation of amines. However, although effective, these prior art catalysts usually require drastic reaction conditions. Typical reactions are carried out at temperatures from about 373 to about 483 K and under pressure from about 2 MPa to about 8 MPa. These severe conditions, especially the significant pressure required, frustrate the use of these catalysts in an industrial setting.
Additionally, prior art processes for the production of carbamate are carried out through a liquid-solid carbonylation process in which solid catalyst is immersed in liquid reactant(s). These processes require high pressures to enable gaseous reactants to reach and react with the catalyst by dissolving into the liquid reactant(s). Also these processes require an involved separation step for recovering the catalyst from the desired product. Developing a new catalytic system that can work at mild conditions and without the need for employing complex separation techniques is a focus of the present invention.
The oxidative carbonylation of amines to carbamate esters catalyzed by a palladium group metal and iodide ion is disclosed in two closely related articles, namely, J Org. Chem. 1984, 49, 1458 and J Chem. Soc., Chem. Commun., 1984, 399, both by Fukuoka and co-workers. They taught that oxidative carbonylation carried out in the presence of these catalysts required a pressure of about 8.3 MPa, which, as mentioned above, is undesirably high for the mass production of carbamates in an industrial setting.
Fukuoka and co-workers studied a liquid-solid carbonylation process in which solid Pd catalyst and solid iodide source are added to liquid alcohol in an autoclave. Carbon monoxide (CO) and oxygen (O2) are pressurized into the autoclave so as to become dissolved in the liquid reagents, in order to reach and react with alcohol and amine on the catalyst. While somewhat effective, this reaction necessarily entails the involved step of catalyst recovery. Particularly, the solid Pd catalyst must be recovered from the carbamate product produced, and, in an industrial setting, this is no easy task. To recover the catalyst, process steps, such as distillation and extraction, must be employed.
Alper and Hartstock, J.Chem. Soc., Chem. Commun. 1141 (1985), disclose catalytic systems including palladium chloride, copper chloride and hydrochloric acid to produce carbamates from amines. This Wacker-type catalytic system, consisting of PdCl2xe2x80x94CuCl2-HCl, is disclosed as being effective at mild conditions (1 atm and room temperature) in the oxidative carbonylation of amines to produce a high yield of carbamate. In this system carbon monoxide (CO) and oxygen (O2) are bubbled through alcohol to which is added, in sequence, PdCl2, hydrochloric acid, CuCl2, and amine. The mixture is stirred overnight, at ambient temperature and pressure, and filtered, and the filtrate is subjected to rotary evaporation. The resulting oil is treated with either diethyl ether or acetone and filtered, and concentration of the filtrate yields the carbamate ester. Further purification is affected by thin-layer or column chromatography (silica gel). Thus, as with the process disclosed by Fukuoka and co-workers, a somewhat complex separation step is involved.
Gupte and Chaudhari, Journal of Catalysis 114, 246-258 (1988), studied the oxidative carbonylation of amines using a Pd/Cxe2x80x94NaI catalytic system. Although effective at producing carbamates, this catalytic system requires the use of high pressures.
The oxidative carbonylation experiments are carried out in a 3 00 cm3 capacity, high-pressure, stirred autoclave. A known quantity of aniline, catalyst, 5% Pd/C, NaI and alcohol solvent are charged into the autoclave. The autoclave is pressurized with carbon monoxide and oxygen to a total pressure of 4.1 MPa. The contents are heated and the reaction started by switching on the stirrer. The reaction is carried out at a constant pressure by supplying a CO:O2 mixture (2:1 ratio) from a reservoir. After two hours, the contents are cooled and products analyzed. The process taught by Gupte and Chaudhari involves liquid-solid carbonylation and therefore requires the burdensome step of separating catalyst from the carbamate product produced.
U.S. Pat. No. 4,976,679 to Okawa et. al. discloses a process for producing carbamate that comprises reacting a primary amine, an organic compound containing a hydroxyl group or groups (e.g. alcohol), carbon monoxide, and molecular oxygen in the presence of catalytic systems containing at least one member selected from copper and copper-containing compounds and at least one halogen selected from iodine, chlorine, and bromine. Thus, Okawa discloses that expensive palladium group metals are not necessary catalysts for the oxidative carbonylation of amines to carbamates. Okawa does, however, teach the process as being carried out in a sealed autoclave wherein the catalyst and amine are placed in the alcohol, and carbon monoxide and air are added to the autoclave and subjected to pressure so as to dissolve in the alcohol and therefore reach and react with the catalyst.
While various processes and catalytic systems for the oxidative carbonylation of amines to carbamates are known, these prior art processes have concentrated on the liquid-solid carbonylation process in solvent, and entail the involved step of separating the catalyst employed from the carbamate produced. This separation step is particularly burdensome in an industrial setting. Furthermore, it takes more than 1 hour for these prior art processes to achieve the desired conversion of reactants and desired yields in a batch mode. Thus, there exists a need in the art for an oxidative carbonylation process converting amines to carbamates at non-drastic conditions, at an appropriate reaction rate, without the need for involved separation techniques to recover the carbamate produced from the catalyst employed.
The process disclosed herein satisfies the need in the art for an industrially viable oxidative carbonylation catalytic system, and is capable of producing carbamates at a significantly higher rate than those processes reported in journal and patent literature. This reaction process takes place via a reaction mechanism that does not involve drastic conditions. Specifically, the catalytic system of the present invention employs Group VIII metal catalysts and/or copper-based catalysts with halide promoters to produce carbamates through heterogeneous oxidative carbonylation at atmospheric pressure and relatively non-drastic temperatures in a gas-solid carbonylation process.
The present invention provides a method for the synthesis of carbamate through gas-solid oxidative carbonylation. Advantageously, this heterogeneous process can be carried out at non-drastic conditions and without the need for a subsequent catalyst recovery step. Various catalytic systems, whether known or heretofore unknown in the art, may be employed. Such catalyst systems may include copper-based catalysts, Group VIII metal-based catalysts, or co-catalytic systems including one or more Group VIII metal-based catalyst and/or one or more copper-based catalyst.
In general, the present invention provides a gas-solid carbonylation process for the production of carbamate ester. This process involves reacting, in the gaseous state, an amine, alcohol, oxygen, and carbon monoxide in the presence of a solid catalytic system including at least one catalyst of the formula MnXm and at least one alkali salt of the formula AZ, wherein M is copper or Group VIII metal, X is a halogen or oxygen, n and m are whole numbers dependent upon the balancing of the valence numbers of M and X, A is an alkali metal, and Z is a halogen.